A hard disk drive typically includes one or more magnetic disks rotatably mounted in association with a spindle and one or more actuator assemblies for positioning a transducer or head relative to data tracks on the surface of each disk. The actuator assembly typically comprises a pivotable arm, a load beam or suspension arm affixed to the pivotable arm, a flexure at the distal end of the load beam and a slider and transducer or head assembly affixed to the flexure. A voice coil motor induces movement of the actuator assembly to position the head relative to the disk surface. Typically, the voice coil motor operates in association with a servo system to provide both gross positioning of the head, i.e., track to track positioning, and fine positioning, i.e., track following. Recently, piezoelectric elements have been incorporated into actuator assemblies to provide fine positioning of the head assembly for track following purposes, rather than only relying upon the voice coil motor for fine positioning.
With the emphasis on making hard drives smaller for numerous applications including portable computers, magnetic disks are not only becoming smaller, but data tracks are becoming increasingly more densely positioned on the disks and the tracks themselves are becoming physically narrower. As a result, maintaining the transducer or head in an accurate track following position for purposes of reading and writing is becoming more difficult. To accommodate increasingly finer adjustments in the position of the magnetic head, dual positionable actuator assemblies have been introduced. In a first mode, a voice coil motor will move the actuator assembly from track to track. In a second mode, the voice coil motor together with a piezoelectric element positioned on the actuator assembly will provide fine positioning of the head assembly. In particular, by supplying a voltage to the piezoelectric element, the piezoelectric element can expand and contract in a controlled manner to adjust the distal end of the actuator arm and thereby accurately maintain the position of the head relative to the tracks on the disk surface.
The dual-stage actuator assembly including both a voice coil motor and a piezoelectric actuator can maintain more accurate position of the magnetic head during track following control than a single-stage actuator assembly, e.g., the voice coil motor alone. However, vibrations due to mechanical resonance modes of the actuator assembly, including the voice coil motor and the piezoelectric actuator, limit further improvement of the head positioning control accuracy. In particular, the resonance modes of the actuator assembly limit the attainable track following servo control bandwidth. Furthermore, the vibration of these resonance modes, excited by air turbulence and gross positioning commands (e.g., seek control commands) executed during fast and/or relatively large track-to-track seeking, is a major source of head off-track position errors.
The piezoelectric material used for the piezoelectric actuator has also been used to detect vibrations and/or shocks affecting the actuator assembly. In U.S. Pat. No. 6,100,628 issued to Huang et al., for example, a dual-purpose, bulk piezoelectric element provides for fine positioning of a head assembly relative to a surface of a disk of a hard disk drive and for sensing vibrations affecting the actuator assembly. Since the same piezoelectric element is used both for actuation and sensing, however, the sensor signal must be extracted from the driving signal applied to the piezoelectric element for positioning the head assembly. Thus, discriminator and/or filtering circuitry is required to separate the sensor signal from the driving signal. This not only increases the system cost, but can also degrade the performance of the sensor when there is feed-through of the driving signal to the sensor signal.